The [C ii] \(158\,\mu \mathrm {m}\)  fine-structure line is the dominant cooling line of neutral atomic gas at moderate densities (\(n_{\rm H}\lesssim 10^4\,\mathrm {cm^{-3}}\) ) and temperatures [1]}. This includes diffuse clouds in the general interstellar medium (ISM) of galaxies as well as regions illuminated by nearby, newly formed, massive stars [2]}. In these regions carbon atoms are rapidly ionized by penetrating far-ultraviolet (FUV) photons \(E<13.6\)  eV) and molecules such as CO are quickly photodissociated. Hence, most of the gaseous carbon is in the form of C\(^+\)  ions. The ground state of C\(^+\)  has two fine structure levels separated by \(\Delta E/k_{\rm B}=91.2\,\mathrm {K}\) . This transition has a critical density of \((2\text{-}6)\times 10^3\,\mathrm {cm^{-3}}\)  for collisions with atomic or molecular hydrogen. Since the upper fine-structure level \(^{2}P_{3/2}\) is easy to excite collisionally at typical ISM densities and as the interstellar carbon abundance is high, the [C ii] \(158\,\mu \mathrm {m}\) line dominates the cooling of the neutral ISM.
